Plasticiser esters are produced by reaction of the appropriate alcohol, typically a C4 to C13 alcohol, with an acid anhydride, frequently phthalic anhydride, cyclohexane dicarboxylic acid anhydride, trimellitic anhydride, maleic anhydride, or with an acid. Acids frequently used are adipic acid, trimellitic acid, cyclohexanoic mono- and dibasic acids, benzoic acid, citric acid and the like. The esterification is typically performed using an organo-metallic catalyst particularly a titanium or tin based catalyst, but many other esterification catalysts like sulfuric acid and para-toluene sulfonic acid are also known. The term crude ester as used herein, means the product of esterification, which will contain contaminants and requires purification. These contaminants can belong to the family of acidic residues, unreacted alcohol or unreacted acid, catalyst residues, water and the contaminants that were already present in the alcohol feed, most of these being so-called monomeric components and showing up in the so-called “light ends” region of the Gas Chromatogram or GC-spectrum of the ester. The esters can also contain byproducts, such as alcohol (di-alkyl)ethers, mono-esters from dibasic acids, alcohol oxo acid esters, hemiacetals and vinyl ethers. These are so-called dimeric components and are often collectively called “ethers” or “intermediates” due to their elution in the Gas Chromatogram or GC-spectrum of the ester between the monomeric light ends and the “trimeric” diesters.
It is known from U.S. Pat. No. 5,324,853 to purify esters of dicarboxylic acids or anhydrides by contacting the crude ester with aqueous alkali such as sodium hydroxide or sodium carbonate. The addition of the water and the alkali hydrolyses and/or neutralises catalyst residues and neutralises any undesirable mono-ester that may be present. The neutralised ester is then typically filtered for the removal of salts such as the alkali salts of the mono-esters, the hydroxide of the organo-metal catalyst such as titanium hydroxide, the oxide of the organo-metal catalyst such as titanium dioxide or tin oxide, and sodium (bi-)carbonate. The alkali used for the neutralization is preferably sodium carbonate or in some instances it may be sodium hydroxide, preferably in an aqueous form. The hydrolysis and/or neutralisation may be followed by injection of carbon dioxide to convert any remaining sodium hydroxide into water and sodium (bi-) carbonate. Finally any excess alcohol and water may be removed by flashing or stripping with a vapour, e.g. with steam or nitrogen, or by a combination thereof.
Example 5 of U.S. Pat. No. 5,324,853 describes the neutralization of an ester obtained from phthalic anhydride and isodecyl alcohol using tetra-isopropyl titanate as an esterification catalyst. The neutralization is effected with a dilute solution of soda ash containing enough soda ash to provide 1.5 to 2 stoichiometric equivalents of sodium carbonate and enough water to provide from 1 to 6 wt % water based on the batch. The temperature for the treatment can be from 70° C. to 120° C. although the acceptable temperature range is said to be from 90° C. to 140° C. U.S. Pat. No. 5,324,583 suggests that carbon may be added before the start of hydrolysis so that decolouring can occur at the same time as the neutralization and hydrolysis. It also describes the addition of clay or filter aid after hydrolysis and before filtration. However, this does not address the problem that there is a tendency for the titanium hydroxide and the salts formed in the neutralization reaction to agglomerate or gel together, rendering it difficult to remove by filtration.
We have found that the purification process described in Example 5 of U.S. Pat. No. 5,324,853 suffers from two disadvantages. Firstly, the levels of excess soda ash used are such that they can lead to undesirably high levels of sodium in the final plasticiser ester. This in turn can cause the plasticiser to initiate undesirable pre-polymerization of isocyanates when it is used as a solvent for the isocyanate in the production of polyurethanes, and may impair the electrical properties when used, for example, in wire and cable insulation. Secondly, on an industrial scale where several hundred tonnes of material are to be filtered, the levels of water used in U.S. Pat. No. 5,324,853 can cause a rapid increase in the pressure drop across a filter leading to a reduction in filtration efficiency and also a reduction in the life of the filter.
The pressure drop in a filter is the difference between the inlet pressure at the filter and the outlet pressure and primarily is the pressure loss over the filter cake. If the pressure drop becomes too high, the filter cake becomes compacted, so inhibiting filtration, and furthermore the filter cake can become difficult to remove. In the process of U.S. Pat. No. 5,324,583 water is removed before filtration by flashing as rapidly as possibly. In an industrial scale process this typically involves performing the neutralization at from 100° C. to 140° C. so that the temperature is such that the water may readily be flashed off.
Plasticiser esters (also termed simply “plasticisers” herein) may be used as solvents for isocyanates in the production of polyurethanes. The plasticiser is typically used in an amount of 20 to 40 wt % of the polyurethane. The isocyanate is dissolved in the plasticiser and this solution is mixed with a polyol to produce polyurethanes. The plasticiser acts as a carrier for the isocyanate and also as a plasticiser for the polyurethane. Typical uses for such polyurethanes include mastics and sealants such as those used in the assembly of glass and in the building, aerospace and automobile industries. It is important that the plasticiser does not adversely affect the isocyanate. We have found that the levels of residual sodium or base that can be present when using the preferred ester finishing technique of U.S. Pat. No. 5,324,853 can cause pre-polymerization of the isocyanate before it reacts with the polyol, leading to undesired gel and sediment formation.
U.S. Pat. No. 6,150,552 discloses a process for the production and purification of tetrahalophthalate esters after reaction of a tetrahalophthalic compound with an alkanol in the presence of a titanate catalyst. The mixture of reactants before the reaction is treated with an accurately calculated amount of sodium carbonate necessary to neutralise the residual sulfuric acid in the tetrahalophthalic anhydride, leftover from its production process. The esterification reaction is completed when the acid number of the reaction mixture is below 1 meq/100 g. After vacuum distillation, water and sodium carbonate are added separately to the stripped product. In the process of U.S. Pat. No. 6,150,552, it is essential to perform the hydrolysis after removal of the excess alcohol, and an accurate dosing of the water and/or sodium carbonate is not given a high importance. Example 1 of U.S. Pat. No. 6,150,552 discloses a batch purification process that employs 7 g of sodium to carbonate for the neutralisation, which corresponds to more than 7 times the stoichiometric amount in relation to the acidity of the crude ester. This level of excess soda ash again can lead to undesirably high levels of sodium in the final ester.
Example 7 of U.S. Pat. No. 6,150,552 is concerned with filtration performance. This example teaches that, in absence of sodium carbonate, more water shortens the time to complete filtration. It also teaches that, in the presence of an equal amount of water that is equivalent to less than 0.72% wt based on the weight of stripped product, filtration time reduces when an amount of at least 1.35 times the stoichiometric amount in relation to the acidity of the crude ester is employed. The experiment that employs 0.1 g of sodium carbonate, which corresponds to only 0.225 times the stoichiometric amount, utilises an amount of water less than 0.72% wt based on the stripped product, and is shown to need a longer time to complete filtration as compared to those experiments employing an amount of sodium carbonate above stoichiometry.
The properties and quality requirements for plasticisers depend upon the use to which the plasticiser is to be put. The requirements with isocyanates have been discussed above. Another important property of a plasticiser is its electrical resistivity, particularly when it is to be used in electrical applications such as for wire and cable insulation. More specifically, the present invention also relates to a process which can be combined with the process of our copending PCT patent application WO 2005021482 to produce a high quality plasticiser ester suitable for use with polyvinyl chloride which is to provide a composition useful for wire and cable insulation and as other electrical insulating material.
Plasticised polyvinyl chloride is widely used for insulation in the electrical and electronic industries and these uses require a high-quality plasticiser ester. For example, a plasticiser having high volume resistivity is required in the electrical field. The resistivity of a plasticised polyvinyl chloride composition may be measured as the Pad Volume Resistivity (PVR). Many people in the industry measure also the resistivity of the plasticiser itself, which is known as the liquid volume resistivity (LVR) of a plasticiser. For several electrical applications like e.g. the electrical insulation of under-the-hood or under-the-dashboard electrical wire and cables in vehicles, plasticisers are preferred to have a high LVR, and a low amount of light ends, especially those compounds that contribute to odour and automotive interior and windscreen fog problems. The electrical equipment in vehicles is becoming more and more complex and sophisticated. Modern vehicles are being increasingly equipped with extra sensors and electrically driven devices. The amount of wiring and cabling necessary for connecting these sensors and controlling and powering these devices continues to increase. Many of these connections are placed out of sight under the vehicle upholstery and relatively close to the outer body, where there is little ventilation and temperatures may be high due to engine heat or exposure of the vehicle to sunshine.
Accordingly, in addition to the low sodium levels discussed previously, plasticisers desirably should have an acceptable odour, and should not cause fogging or the creation of a light scattering film on the innerside of car windshields; they should also be resistant to ultra violet light. The plasticiser should contain only minimal amounts of volatiles or light ends in order to have a low odour level both during its processing and in its final application.
U.S. Pat. No. 5,880,310 is concerned with purifying plasticiser esters to produce materials with high liquid volume resistivity as measured by Japanese Industry Standard JIS K-6751. U.S. Pat. No. 5,880,310 obtains high volume resistivity of a plasticiser ester by blowing carbon dioxide into the crude ester that has been neutralised with sodium hydroxide to convert residual alkali into a (bi-) carbonate; recovering any excess alcohol, typically by steam stripping; and then by adding a filter aid to the neutralized and stripped ester followed by fine filtration and adsorption treatment. This process, however, uses excess sodium. Furthermore the process does not perform the neutralization in the presence of a filter aid and there remains a tendency for the products of hydrolysis of the titanium-containing catalyst to agglomerate and impair filterability.
The present invention provides improvements in the purification of plasticiser esters, and in particular improvements that provide plasticiser esters that may be used as solvents for isocyanates in the production of polyurethanes with a reduced tendency to cause the isocyanate to pre-polymerise. The invention is also concerned with improving the filterability of the esters. The invention is also aimed at providing plasticiser esters of an improved purity that are particularly well suited for use in PVC electrical insulation for high resistivity products.